Spiral staircase



FIG. 1 is a perspective view of a spiral staircase, showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a greatly enlarged view thereof;

FIG. 4 is a greatly enlarged top plan view of one of the steps thereof; and

FIG. 5 is a greatly enlarged bottom plan view of one of the steps thereof. 

The ornamental design for spiral staircase, substantially as shown. 